1. Field of the Invention
The present invention relates to a light measuring method and a light measuring apparatus for automatically measuring the brightness of an object and a place (scene), etc., and particularly to a light measuring method and a light measuring apparatus used in an automatic exposure control apparatus of a camera.
2. Description of the Related Art
Most of cameras recently produced are equipped with an electronic shutter (automatic exposure control apparatus), therefore a measuring of brightness of an object and a background as well as a determination of an exposure are done in a totally automatic manner. An electronic shutter consists of, as known well, light measuring elements for producing such electric outputs as corresponding to the brightness of an object and a background and a signal processing circuit for processing output signals of the light measuring elements, and many of such electronic shutters have all of their signal processing circuits made of analog circuits, but in the case of an SLR camera (a single lens reflex camera), it employs a TTL light measuring system and needs a long time memorizing of the light measuring information, thus circuit portions relating to a computation processing and a memorizing of an electronic shutter for an SLR camera are digitalized except the circuitry for compressing the output signals of the light measuring elements. However, although the processing of light measuring signals with a digital circuit allows a computation with a high level of accuracy and at the same time it is advantageous for memorizing the light measuring information, when a light measuring is done under an illumination by an artificial light source, some counter-measure needs to be taken to prevent an influence of flicker of such light source from affecting the light measuring.
FIG. 4 shows an arrangement of important parts of a light measuring apparatus in a conventional electronic shutter for an SLR camera. In this diagram, SPA is a light measuring element positioned at the center of a photo-taking image plane, and SPB is a light measuring element for measuring light at a periphery of a photo-taking image plane, while OP is an operational amplifier to amplify output signals of these light measuring elements SPA and SPB, then LD is a logarithmic compression element for logarithmically compressing the output signals of the light measuring elements, and AD1 is an AD converter of a double integration type for converting the output of the operational amplifier to digital signals, then CKT is an operational circuit. At the input terminals of the operational amplifier, analog switches FGA and FGB, which are connected in series respectively to the light measuring elements SPA and SPB and are made of MOSFETs, are connected, and are so arranged that the switch FGA only is turned on by a control signal impressed onto a gate .phi.A when a spot light measuring is done, and both of the switches FGA and FGB are turned on by control signals impressed onto gates .phi.A and .phi.B when an averaging light measuring is made. Also, analog gates EGA and EGB are provided in parallel to the light measuring elements SPA and SPB, and these analog gates EGA and EGB are so made as being turned on and off in a mutually compensating manner with the analog switches FGA and FGB which are connected in series to them respectively. That is, when a control signal is impressed onto the gate .phi.A and the analog switch FGA is turned on, a control signal which makes the analog gate EGA non-conductive is impressed onto the gate .phi.A, then the analog gate EGA is turned off. These analog gates EGA and EGB are to release the voltages generated at the light measuring elements SPA and SPB when the analog switches FGA and FGB are turned off, for short-circuiting the light measuring elements. While the AD converter AD1 has a function of converting the output signal of the operational amplifier OP to a digital signal, it needs to have a function of completely eliminating an influence of flicker of an artificial light source in a light measuring under such light source.
FIG. 5 is a diagram to show the principle of an AD conversion in the AD converter AD1 of a double integration type used in a conventional light measuring apparatus. In this diagram, V.sub.A and V.sub.B are signals obtained by logarithmically compressing and amplifying the output signals of the light measuring elements SPA and SPB, while V.sub.A ' and V.sub.B ' show signals by a discharge from an integration capacitor housed within the AD converter AD1. Also, T.sub.1 is an integration time and T.sub.2 and T.sub.3 show an inverse integration time, wherein the integration time T.sub.` is set at 10 m.sec. which is equivalent to one cycle of flicker of such artificial illumination light source as a fluorescent lamp (provided that a driving power source for that light source is assumed as commercial alternating current of 50 Hz or 60 Hz). In this diagram, when an AD conversion of the input signal V.sub.A is started at the time t.sub.1, the output V.sub.A of the light measuring element SPA is integrated for a period of time of T.sub.1 until the time t.sub.2 then a discharging will be made from the integration capacitor housed within the AD converter AD1 during a period of time of T.sub.2 that is from the time t.sub.2 to the time t.sub.3. The output V.sub.B of the light measuring element SPB has also, after being integrated during the period of time of T.sub.1, a voltage drop V.sub.B ' by the inverse integration through the discharge from the integration capacitor after the time t.sub.2 continued till the time t.sub.4 by the inverse integration. Digital signal outputs corresponding to the input analog signals V.sub.A and V.sub.B become such pulse train as equivalent to the lengths of the periods of time T.sub.2 and T.sub.3 and are impressed onto the operating circuit CKT.
An AD converter of a double integration type as mentioned above has a drawback that when a number of light measuring elements is n, a period of time, (10.times.n) m.sec. will be needed for digitally converting the output of each light measuring element as the minimum time of 10 m.sec. is needed for that digital conversion, thus a length of time required for light measuring is too long. Therefore, a camera housing an electronic shutter of such light measuring system as mentioned above cannot photograph an object moving at a high speed.